An optical communications system that uses optical fibers requires a coupling optical system that converges transmitted light that diverges from a light source or the optical output end surface of an optical fiber and enters one side of the coupling optical system so that the transmitted light is concentrated and collected to efficiently enter the light incident end surface of an optical fiber.
Conventional coupling optical systems used in optical communications that include a ball lens (spherical lens) have been commonly known. An advantage of using a ball lens is that installation is easy because alignment is unnecessary. However, there is a disadvantage in that transmitted light is difficult to efficiently concentrate on the light incident end surface of the optical fiber that is receiving the light due to a large spherical aberration. Consequently, there is a problem in coupling optical systems that use a ball lens in that enhancing the coupling efficiency is difficult.
Coupling optical systems for optical communications that use an aspheric lens have also been proposed, for example, in Japanese Laid-Open Patent Application H09-61665. Light transmitted by the coupling optical system can be collected on the light incident end surface of the optical fiber by suppressing the spherical aberration by using an aspheric lens. Thus the coupling efficiency can be improved in comparison to using a ball lens.
However, a coupling optical system for optical communications that uses an aspheric lens has a problem of large deterioration in the coupling efficiency when environmental conditions or operating conditions of the system change due to the position of the light collection point of the transmitted light moving significantly in the optical axial direction. For example, significant movement may occur when the wavelength of light incident on the coupling optical system is different from the design wavelength due to a change in the system, or when the wavelength of the light incident on the coupling optical system fluctuates due to temperature changes in the working environment that may occur, for example, when using a semiconductor laser as the light source. Further, an aspheric lens made of common plastic has a higher chance of reducing the coupling efficiency because the refractive power varies a lot due to temperature changes in the working environment.
Applicants of the present application have previously described, for example, in Japanese Patent Application 2005-67009, an optical element having variable refractive power formed of a liquid that includes a dispersion of microscopic particles between two transparent media. This variable refractive power element is expected to be used in various optical systems because the refractive power of the optical element, as well as the reciprocal of the refractive power (i.e., the focal length of the optical element), is variable according to the switching of an electromagnetic field applied to the liquid that varies the migration of the microscopic particles of the dispersion within the liquid.